Halftone color separations are used in reprography as copy originals for preparing offset or letterpress printing plates. Before the printing plates are exposed, the color separations are inspected with the aid of a color proofing process to determine whether the ultimate printing result will be a tonally accurate reproduction of the original.
Such proofing processes use radiation-sensitive recording materials on which an image is produced by differences in the tackiness of exposed and unexposed areas of the radiation- sensitive layer. For example, German patent DE-C 12 10 321; U.S. Pat. Nos. 3,620,726; 3,582,327; 3,649,268; 4,356,253; 4,892,802; and 4,948,704 disclose a reproduction process in which a tacky photopolymerizable recording material, comprising a support and a photopolymerizable layer having at least one addition-polymerizable monomer and a photopolymerization initiator, is hardened by imagewise exposure, causing the exposed image areas to lose their tackiness.
The latent image is made visible by the application of suitable toners that adhere only on the unexposed tacky areas and can be removed, after application, from the exposed nontacky image areas. This process yields positive, optionally colored images of the original. These images resemble in appearance the images that would be produced by printing inks.
Negative images of the original are obtained by using the photosensitive materials described in U.S. Pat. Nos. 4,243,741; 4,346,162; and 4,604,340. The photosensitive components in these recording materials are either a dihydropyridine compound or a system of a dihydropyridine compound and a hexaaryl bisimidazole compound.
The toners, which comprise predominantly finely divided powders, are applied by dusting them on the imagewise exposed surface, as described, for example, in U.S. Pat. Nos. 4,215,193 and 3,909,282. In another embodiment, the toner can be loosely bonded on a special support and transferred by bringing this support into contact with the imagewise exposed layer. Such pigmented support layers are described in U.S. Pat. No. 4,806,451 and DE-C 39 41 493.
The radiation-sensitive recording materials usually comprise a support, a radiation-sensitive layer, and a cover sheet. This cover sheet is stripped off and the material is laminated on an image support before imagewise exposure. The support can be stripped off before or after exposure, depending on the material.
The printing process causes mechanical dot growth. This dot growth must obviously be taken into account in a color proofing process to assure a tonally accurate reproduction in the ultimate printing result.
The processes described in DE-C 12 10 321; U.S. Pat. Nos. 3,620,726; 4,243,741; 4,346,162; 4,604,340; 3,582,327; 3,649,268; 4,356,253; 4,892,802; and 4,948,704 produce optical dot growth due to the thickness of the photopolymerizable layers. This corresponds to mechanical dot growth in conventional printing processes.
The development of modern planographic printing processes has reduced mechanical dot growth. However, the above-described color proofing processes are still limited in their capability for making a tonally accurate reproduction of the ultimate printing result in modern planographic printing processes.
Therefore, the graphic arts industry demands color proofing processes with especially low optical dot growth for modern planographic printing.
Optical dot growth is reduced, as discussed in U.S. Pat. No. 4,889,787, by reducing the thickness of a transparent auxiliary layer that is proximate to the radiation-sensitive layer. Another possibile approach to reducing optical dot growth in color proofing processes, is to use titanium dioxide in a first layer laminated on an image carrier, as described in U.S. Pat. No. 5,176,973. The layer containing titanium dioxide can be applied either alone as the topmost layer on the image carrier or together with the first radiation-sensitive layer on the image-side surface of the image carrier.
Color proofing processes using image carriers with low optical dot growth are described in U.S. Pat. Nos. 4,579,810 and 5,075,196. The latter discloses using a layer comprising titanium dioxide and a hydrophilic colloidal binder on a paper base coated with a polyolefin.
U.K. Patent 13 39 045 describes a support for photographic emulsions with two layers on the image-side surface and having an inorganic white pigment essentially only in the outer layer.
However, all of these developments are inadequate for the low dot growth required in the above-described color proofing processes.
Therefore, a problem involved in this invention is to propose monochrome and polychrome color proofs having low optical dot growth and a means and process for their preparation. The radiation-sensitive layer of the radiation-sensitive recording material will remain unchanged to preserve its utility in color proofing processes with a standard optical dot growth in the range between 19% and 25% for a 50% dot.